Dichloroethane radiolysis

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

In a later paper72 they studied the ps and ns pulse radiolysis of pure liquid 1,2-dichloroethane (DCE), and observed a broad visible absorption with a miximum around 550 nm. This absorption decayed by first-order kinetics with tVl = 0.2 ns in ps pulse radiolysis and ty2 = 200 ns in ns pulse radiolysis. In the latter there is an additional peak at 270 nm, which is formed during and after the pulse and decays by seocnd-order kinetics with 2 kls = 2x 107cms 1. 

The positive charge of solvent radical-cations transfers to solute molecules in halogenated hydrocarbons such as chloroform and dichloroethane. However, only few studies have been made on the radical cations of polymers in solution. Tanaka et al. observed the dimer cation of the biphenyl group or the pyrenyl group of the polymers in the pulse radiolysis of PVB and PVP in 1,2-dichloroethane [49]. The absence of the monomeric cation is due to the rapid intramolecular dimerization of the radical cations of the side groups of the polymers. Irie et al. observed two kinds of intramolecular dimer cations in the... 

The dynamics of reversible onium ion formation has been studied by generating carbenium ions in the presence of nucleophiles using pulse radiolysis or flash photolysis, and following the rate of disappearance of the carbenium ions by UV. As discussed in Chapter 2, the kinetics of reaction of various electrophiles with nucleophiles obey a general reactiv-ity/selectivity relationship. The rates of reaction of various nucleophiles with carbenium ions are summarized in Table 9. These rates often approach diffusion controlled limits (k 10 ° mol-,-L-sec l). The rates are slower for less nucleophilic and less electrophilic compounds, and are particularly slow with sterically hindered amines such as lutidine (2,6-dimethylpyridine) [63]. Solvent effects are minimal when the reactions are diffusion controlled, although tributyl amines react slower with carbenium ions in more nucleophilic dichloroethane than in methylene chloride. 

With the objective of oxidizing the fullerene core, radiolysis of any chlorinated hydrocarbon solvent provides the means of forming strongly oxidizing radical species [71]. For example, the radiation-induced ionization of dichloroethane (DCE) yields the short-lived and highly reactive solvent radical cation. In general, the electron affinity of [DCE] + is sufficient to initiate one-electron oxidation of the fullerene moiety (Eq. 6) [72-76]. 

Radiolysis studies on chloro-compounds have included those on chlorinated methanes - - - , 1,2-dichloroethane , chlorotriphenylmethane - , chloroolefins , benzyl chloride - , a-chlorotoluene , and chloroacetic acids . Recoil tritium reactions with dichloromethane and chloroethane have also been reported. 

Figure 2.5. Kinetic traces of AO.D.480 during the pulse radiolysis-laser flash photolysis experiment of t-St (5 x 10-3 M) in the absence (a) and presence (b) of ANS (1.0 M) in Ar-saturated 1,2-dichloroethane.

Figure 2.7. Transient absorption spectra observed at Ons, 100 ns, and 1 ps after the 8-ns electron pulse, and transient fluorescence spectrum of TMB + observed at 300 ns after the electron pulse during pulse radiolysis-laser flash photolysis of TMB (lx 10-2M) in Ar-saturated 1,2-dichloroethane. Excitation wavelength, 532 nm. Laser pulse energy, 140 mJ pulse-1.

The transients formed from phenol (irradiation at 266 nm in ethanol) have been identified as solvated electrons, phenoxyi radicals (an absorption around 400 nm) and the triplet state of phenol (450 nm)". The formation of phenoxyi radicals and hydrated electrons display a low-frequency/high-field absorption and a high-frequency (low-field) emission polarization pattern generated by a radical pair mechanism. Phenoxyi radicals have also been observed following electron transfer from phenols (as solutes) to molecular radical cations of some non-polar solvents (cyclohexane, n-dodecane, 1,2-dichloroethane, n-butyl chloride). This study used pulsed radiolysis and the formation of the phenoxyi radicals is thought to involve Scheme 1. 

The one-electron oxidation of a wide range of compounds has been investigated in this way, including phenols and metallotetraphenylporphyrins. One-electron oxidation of metallotetraphenylporphyrins is also achieved in 1,2-dichloroethane, but in this case pyridine is added as a base to prevent demetallation of the complexes by HCl which is a radiolysis product. 

Diphenyl sulfide radical cation has been reported [116] to absorb in the visible with = 780 nm for the species formed by gamma radiolysis of Ph2S in 1,2-dichloroethane at 77 K. Laser flash photolysis of 46 in acetonitrile provides Ph2Ss which shows absorption bands at 330 and 750 nm [117]. 

Some evidence, in pulse radiolysis, has been obtained (6, 35) for the formation of the cation of diphenyl, p-terphenyl, anthracene, and trans-stilbene in dichloroethane and other chlorinated solvents, and for the p-terphenyl cation in chloroform (6). Some of these cation spectra are shown in Figure 2. The diphenyl solution shows absorption band at 690 m/a and 380 m/m. The diphenylide anion bands in the alcohols and in ethers are at 630 mp and 405 m/, so that the cation bands seem to be distinguishable. Anthracene in dichloroethane shows a band at 430 m/, ... 

Figure 2. Absorption spectra of aromatic cations in chlorinated alkane solvents, formed in pulse radiolysis. (a) Diphenyl in 1,2-di-chloroethane. The spectrum was obtained 2 yjsec. after the electron pulse. (b) p-Terphenyl in 1,2-dichloroethane. The spectrum was obtained 4 nsec, after the electron pulse. The band at 420 m may contain a small contribution from a long-lived species produced by the radiolysis, (c) trans-Stilbene in 1,2-dichloroethane, in 1,1,2-tri-chloroethane, and in 1,1,2,2-tetrachloroethane. Spectrum was obtained 1 fisec. after the electron pulse...

Radical cations of water insoluble porphyrins were studied in chlorinated organic solvents. Pulse radiolysis in dichloroethane led to production of the radical cations of several metalloporphyrins, but an additional process was observed in the case of Mg" and Zn" complexes. It was found that this second process was due to demetallation of these complexes by the HCl formed in the radiolysis. 

H. Levanon and P. Neta, One-Electron Oxidation and Demetallation of Metalloporphyrins and Chlorphyll a in Dichloroethane Solutions as Sudied by Pulse Radiolysis, Chem. Phys. Lett., 70 (1980) 100. 

Since a requirement of the SnI mechanism is the reaction of the carbonium ion with nucleophiles in a fast step subsequent to rate-determining ionization, it would be desirable to have independent data relative to the trapping of carbonium ions by nucleophiles. Triarylmethyl cations react with nucleophiles at rates sufficiently slow to be studied by conventional means, and have provided much information concerning the effectiveness of various nucleophilic species. This will be discussed in the next section of this chapter. The simpler carbonium ions are more reactive, and special techniques must be employed to determine their extremely rapid rates of reaction with nucleophiles. Benzyl cation has been generated by pulse radiolysis in 1,2-dichloroethane, and the absolute rate constants for its reaction with methanol, ethanol, bromide, and iodide ion measured.The second-order rate constants for this group of nucleophiles fall in the range 10 -10 sec ... 

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